Originally Posted by
Ex Pres
UN-72 (and earlier) had removeable cups, which made them great for adapting to French threaded BB and the like. In true Shimano style, they took that adaptability away with the 73 series.
I believe one of the reasons for the redesign between the 72 and 73 series bottom brackets was the chronic creaking problem between the cartridge shell and the inside of the cup with the loose driveside cup on the 72's. The plastic nds cup on some of their bb's was also there to quiet them down.